Scientists believe that autism stems from a combination of genetic and environmental factors. Research done with twins reveals a strong family connection. If one identical twin has autism, the other twin has a 60 to 90 percent chance of also having the condition (in non-identical twins, the rate is about 3 percent). In families with one autistic child, the chance of having a second child with the condition is about 2 to 8 percent -- 75 times greater than among the general population. Also, members of families with autistic children are more likely to have language delays and social difficulties, as well as mental disorders.

Scientists believe that not just one, but a combination of as many as a dozen genes is to blame for autism. Mutations in these genes can make a child more susceptible to autism, or can lead to specific symptoms of the condition. Some of the genes scientists have isolated are HOXA1 (involved in brain structures and nerves), RELN (involved in connections between nervecells), and GABA pathway genes (involved in helping nerve cells communicate with one another).

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These genes likely set the stage for autism, but it's possible that environmental factors actually trigger the condition. A number of environmental factors have been linked to autism, from viral infections to exposure to chemicals such as mercury, lead or polychlorinated biphenyls (PCBs -- a group of chemicals that were once used as lubricants and coolants). Some research has suggested that prenatal exposure to substances such as thalidomide (a drug used in the 1950s and '60s to treat morning sickness, used to treat cancer) or valproic acid (a drug used to treat epileptic seizures) can cause a child to develop autism.

In 1998, a British study by Dr. Andrew Wakefield cast international attention on one potential environmental culprit: childhood vaccines. His small study suggested that the measles-mumps-rubella (MMR) vaccine caused an infection in the intestines, which led to the developmental and gastrointestinal disorders seen in autism. Because children are vaccinated at around the same age as autism is diagnosed, the theory that vaccinations were to blame gained popularity. In the years following the study, however, many of Dr. Wakefield's co-authors disavowed the results, and it was revealed that Dr. Wakefield had a financial motive for linking vaccines to autism. Finally, in 2011, the study was deemed fraudulent by the medical community after it was discovered that Dr. Wakefield falsified data about the children in the report.

Adding to the questions surrounding vaccines was other research indicating that exposure to thimerosal, a mercury-based substance that was once used as a vaccine preservative (specifically in the diphtheria, tetanus, pertussis, Haemophilus influenzae type b (Hib) and Hepatitis B vaccines), could affect brain development and trigger autism. However, thimerosal is no longer present in vaccines, and autism rates haven't dropped.

In 2004, the Institute of Medicine completed a thorough review of all the evidence related to vaccines and autism, and concluded that there was no apparent link between thimerosal or the MMR vaccine and autism. Several other large studies have echoed those conclusions.

We'll look at the brain and autism next.

Theory of Mind

In 1995, Dr. Simon Baron-Cohen of Cambridge University proposed a new theory about autism. He suggested that many individuals with autism suffer from "mindblindness" -- that is, they are unable to understand that other people have their own unique thoughts and emotions. It is this inability to empathize and relate to differences in others' way of thinking that results in the communication and social difficulties that people with autism experience, according to Dr. Baron-Cohen.